The use of fuel flow systems for aircraft is well-known in the art. Such fuel flow systems typically include a turbine flow transmitter which is placed in the fuel line and provides an output frequency which is a function of the volume of fluid passing through the transmitter. This frequency is subsequently converted into a flow rate which is then displayed on an indicator. The flow rate can either be displayed in a volumetric flow rate, indicating gallons per unit time, or as a mass flow rate, indicating pounds per unit time. The relationship between the mass rate and volume rate being the density of the fuel represented in terms of pounds per gallon.
Known fuel flow systems generally have a very high error in their output because of numerous variations in the system. The system is usually calibrated in accordance with particular nominal values of density and temperature. However, the actual values differ greatly from the assumed nominal values. The difference occurs because the fuel used may be one which is different from that assumed as the nominal fuel. Also, there exists a great batch-to-batch variation in density within particular fuels. Furthermore, the variation in temperature from the nominal value assumed is quite drastic. Thus, the likelihood of the fuel temperature remaining at a constant value is remote. Fuel temperatures can readily range from 40.degree. F. to 200.degree. F. at the flow meter, and in certain cases the temperatures can even approach a low extreme of -40.degree. F.
In order to reduce the errors in fuel flow sytems, the prior art has provided for compensated systems which can compensate for density and/or temperature. Most of the compensated fuel flow systems provide compensation within the electronic circuitry whereby the circuitry reacts to various temperature changes or variations in the density of the fuel. Such compensation means, when placed in the electronic circuitry, are not very accurate because they do not truly reflect the exact variations in temperature or density of the fuel itself.
In addition to the aforementioned sources of errors, an additional source of error can result from the particular flow transmitter being utilized. Typically, the flow transmitter produces a fixed number of pulses per volume of fluid passing therethrough. The functional relationship of pulses per gallon is generally referred to as the K factor. The value of the K factor is also utilized in the electronic circuitry to convert the pulses to a volumetric flow rate. If, however, the actual K factor of the flow transmitter does not equal the nominal K factor which is utilized in the electronic circuitry, volumetric flow rate determined will be in error. This error will be compounded during subsequent operations, such as integration, and will produce confusing and erroneous output values.
In prior art devices, in order to correct the K factor it was necessary to structurally modify the flow transmitter by adjusting its turbine blades to produce the desired K factor. Therefore, each time a flow transmitter was inserted into a particular flow system, it had to first be sent back to the manufacturer for proper adjustment and modification of its turbine blades in order to produce the desired K factor. These structural adjustments and modifications resulted in an expansive system and frequently caused time delays as well as causing restricted use of such flow systems and costly servicing thereof.